The California-headquartered venture capital firm Andreessen Horowitz -- whose portfolio includes AirBNB, Pinterest, Skype, Twitter and more -- is in the midst of raising a colossal $1.5 billion for its fourth fund in a total of six years, according to a report byFortune.

In 2012, the company also raised $1.5 billion, comprising $900 million in early-stage investments and a $600 million "parallel fund" for bigger opportunities. The new fund will reportedly include $1 billion in early-stage investments and a $500 million parallel fund.

Sources told Fortune that the fund is already oversubscribed and therefore unlikely to accept new partners.